Experimental Facility for Testing Unconventional Reservoirs: Effect of Geomechanics on Multiphase Flow Properties

Abstract

ABSTRACT: Hydrocarbon production induces reservoir depletion changes the in-situ stress state. This process alters the effective stress and deforms the reservoir rock, changing the multiphase flow properties. An integrated laboratory testing facility was developed to investigate the effects of triaxial stress on the governing mechanisms of multiphase fluid flow. The paper describes the experimental facility for testing unconventional reservoirs and the system performance. The multiphase triaxial system is composed of a high-pressure triaxial cell, an axial loading system, a multiphase pore pressure circuit, a fluid phase separation system, and a logging system. The setup is installed inside an oven to maintain isothermal conditions. This study presents the initial results of testing on sandpack and Berea sandstone specimens. The experimental procedures and testing facility design can be applied across many disciplines which require two phase flow assessment until changing effective stress conditions including: geothermal processes, radioactive waste repositories, and carbon capture and storage operations. 1. INTRODUCTION Hydrocarbon reservoirs are dynamic systems that change considerably through their life cycle. Deformation of the reservoir rock occurs in response to changes in the effective stress during production and stimulation operations. Effective stress is defined as the difference between the overburden or total stress and pore pressure (Terzaghi 1943). Primary hydrocarbon production reduces the reservoir pore pressure, altering the in-situ stress state. The effective stress changes cause deformation in the reservoir rock mass, affecting single and multiphase flow properties (e.g., porosity, absolute permeability, and relative permeability). Porosity is the fraction of void space over the total volume of the rock, while absolute permeability is the total flow capacity of the rock. Relative permeability is defined as the mobility of a continuous phase (effective permeability) to the absolute permeability of the rock. It also can be understood as flow interference. The pores occupied by one phase are unavailable for flow by the other (Persoff and Pruess, 1995). Relative permeability is also known to be a dynamic property because it is affected by several factors, such as stress, wettability, interfacial tension (IFT), and saturation history (Hamoud 2012, Jahanbakhsh et al., 2016). Stress-dependent flow properties are essential to understanding the flow behavior and fluid distribution within reservoirs. Jones (1975) studied the evolution of permeability on natural rocks at various levels of compressive loading and found that permeability was greatly reduced by increases in confining pressures equivalent to magnitudes observed during reservoir depletion. Zhu and Wong (1997) conducted triaxial compression experiments to investigate the influence of stress and failure modes on permeability using Berea sandstone. They found that a drastic decrease in permeability was triggered by shear-enhanced compaction caused by grain crushing and pore collapse. Moghadam et al., (2016) experimentally measured the influence of mean effective stress on shale samples and determined that steady-state gas-permeability is a function of effective stress. Haghi et al., (2020) studied the multiphase flow properties of Berea sandstone at three effective stress levels. The experiments were conducted at steady-state conditions for nitrogen and water. They showed that with the increase in effective stress, porosity and permeability decreased from 13.16% and 58 mD to 12.24% and 36 mD, respectively. The effect of geomechanics on fluid flow is important for financial planning, production forecast, recovery estimations, and underground storage (Yin et al., 2017; Yang et al., 2017; Agheshlui and Matthai, 2018; Sinha et al., 2020). This study presents an experimental facility that provides an integrated approach to investigate the effects of triaxial stress on the governing mechanisms of multiphase fluid flow.